Lift axle valve units are used for adapting the number of the load carrying axles of the utility vehicle to the actual needs. For an empty utility vehicle or a load of the vehicle below a threshold one or a plurality of lift axles are lifted so that these axle(s) have no contact with the road. When exceeding a threshold of the load, the lift axle valve unit automatically lowers at least one lift axle for distributing the load to an increased number of axles. This is done by deaerating lift bellows which in the aerated state lift the lift axle. At the same time with deaerating the lifting bellows additional air suspension bellows of the lift axle are aerated for supporting the axle load of the lift axle. The intention for a change of the number of the load carrying axles of the utility vehicle is also to minimize wear at the load carrying axles and/or a provision of an improved brake behavior for varying loads. It is also possible to change the number of the load carrying axles in case that fees as tolls depend on the number of axles being in contact with the road. The lift axle valve unit might also be responsible for changing the pneumatic pressure of the air suspension bellows of permanently active axles as well as of an activated lift axle to a desired distribution for carrying the axle loads. The lift axle might also be additionally used for keeping a required maximum of the brake force produced by a brake device with an increased load (and corresponding increased normal force between the wheels and the road) below an upper limit value. This is due to the fact that excess brake forces cannot be produced by brake force actuators or these excess brake forces cannot be withheld for longer time periods. Accordingly, with an increase of the load without the additional lowering of the lift axle the brake device would be overloaded. It might also be impossible to produce the required brake force which might result in an increased braking distance.
DE 199 05 113 B4 discloses the option to intentionally lift a lift axle for a short time interval. This is done for purposefully overloading a drive axle of the utility vehicle in order to support a driveway of the vehicle on slippery ground.
A plurality of different lift axle valve units providing at least some of the aforementioned functions is known from the prior art. As some examples for design of lift axle valve units the publications US 2011/0101257 A1, EP 0 170 794 B1, DE 44 10 892 C2, DE 43 14 994 C1, DE 40 37 461 C1 (corresponding to EP 0 487 928 B1), DE 36 38 849 A1 and DE 89 11 010 U1 are cited. Known lift axle valve units are controlled manually, automatically, mechanically, pneumatically and/or electronically, wherein the control might base on a closed or open loop control.
DE 10 2004 051 812 A1 (corresponding to US 2007/0246999 A1) discloses a pressurized air supply device including an air treatment module. A heating device is located close to an electronic control unit of the air treatment module. The designated function of the heating device is not further specified in the patent application. Apparently the heating devices serves for keeping the temperature of the electronic control unit above a temperature limit or to protect an air dryer and/or a pressure regulator against freezing. The pressurized air supply device also includes a lift axle module which is located separately from the air treatment module and located distant from the heating device.